British researchers say they have pinpointed the evolutionary moment that jump-started the birth of intelligence 550 million years ago.

If you have the intellectual capacity to read this sentence, then perhaps you have a wriggly ancient sea creature to thank for it.

British researchers say they have pinpointed the evolutionary moment that jump-started the birth of intelligence and complex thought: 550 million years ago, when an underwater invertebrate experienced a “genetic accident” that led to an explosion of new genes.

But the findings, published in two complementary papers by the journal Nature Neuroscience, also suggest that for humans — a descendant of this sea creature — higher intelligence ultimately came at a price: mental illness.

“What we did was prove that these genes that arose 550 million years ago are the genes that made our behaviour much more sophisticated,” said Seth Grant, a professor at the University of Edinburgh and one of the co-researchers. “But the very same genes that made us more intelligent and sophisticated are the genes that are damaged in people with learning disabilities, schizophrenia, autism and other brain diseases.

“If those genes aren’t working, then that’s when that individual will suffer from some brain disease,” he said. “That’s the price. That’s the risk.”

Grant said it has become widely accepted that half a billion years ago, a sea creature underwent a sudden duplication of genes — perhaps due to an extreme environmental event that favoured the survival of those carrying certain genetic mutations.

This human ancestor may have been something like the Pikaia gracilens, a five-centimetre eel-like creature that once swam off the coast of what is now western Canada. The Pikaia is the earliest known animal with a primitive backbone — this was proved by researchers at the Royal Ontario Museum and University of Cambridge in another recent paper — and a common ancestor to all vertebrates, including humans.

The result of this genetic accident was a new array of genes and increased diversification; think of a Lego set that suddenly has more pieces to use for building.

But what did these new Lego pieces do? Grant and his co-researchers set out to investigate the role of the new brain gene, specifically the GluN2 family of genes (which controls memory and learning) and Dlg genes, mutations of which have been associated with schizophrenia patients, according to the paper.

The researchers tested these genes by deleting them in genetically engineered mice. The rodents were then put inside a box with a touchscreen wall, similar to an iPad. A battery of tests was then applied to both the modified mice and regular mice to comparatively test their ability to think, learn and adapt.

For example, an image might appear on the screen and the mouse would be given a food reward if it poked the image with its nose. The test was repeated with the same image in new locations and eventually, the food reward was taken away. Did the mouse keep poking? Researchers found that mice with the brain genes could adapt and learn; those missing the genes could not.

Researchers applied the same touchscreen tests to human subjects missing the Dlg2 gene: patients with schizophrenia. Their test results were the same as the mice without the gene.

“That automatically tells you that schizophrenia and cognitive impairments are not in any way linked to humans,” Grant said. “It’s quite an ancient psychological phenomenon that would have appeared in many other species that had the same type of mutations.”

For Grant, this is more than just “blue sky research” — he believes these experiments could also have positive implications for medicine.

Grant said his research is the first to apply these cognitive tests to humans and mice with a shared brain gene mutation — and show the same effect. Perhaps this model of testing can now be used to not only help diagnose mental illnesses, but also develop drugs to treat them, he said.

“One of the greatest scientific problems is trying to understand how intelligence and complex behaviours arose during evolution,” Grant said.

“By exploring this very deep and fundamental mystery, we actually might be able to help mankind with respect to brain diseases.”